Electrospray ionization is an electrical phenomenon. To aid in the understanding of the electrical phenomena, an equivalent circuit is often drawn. In an equivalent circuit, electrical components of known properties are used to simulate the behavior of the actual circuit. In the equivalent circuit, one chooses components that have a current-voltage behavior similar to that of the individual elements in the actual circuit. In this paper, we have analyzed each of the elements in the electrospray circuit with respect to its current voltage curve so that an appropriate equivalent circuit can be drawn. The achievement of a useful equivalent circuit serves the purpose of separating the electrical effects of the various circuit elements so that they may be studied and understood individually. This is particularly advantageous in a system with so many interactive elements as electrospray ionization.
The accepted circuit diagram for the electrospray ionization process is shown in FIG. 1. The voltage from the power supply 3 is connected to a metal contact 4 through which the analyte solution flows. The metal contact 4 may be the metallic electrospray needle 5 itself, or a metallic union 6 that joins the needle 5 to the capillary tubing 7 that supplies the analyte. The connection between the metal and the analytical solution is essentially electrochemical. The analytical solution issues from the spray tip in the form of charged droplets 8. The droplets have the same charge sign as the pole of the power supply 3 that is connected to the electrochemical contact. The charged droplets 8 are attracted across an air gap 9 to the counter-electrode 10 where they are neutralized. The connection between the counter-electrode 10 and the other pole 11 of the power supply 3 completes the circuit. A small orifice 12 in the counter-electrode allows some of the ions from the solution to enter the vacuum chamber of the mass spectrometer (not shown) for mass analysis. The fraction of charge that enters the orifice is also neutralized eventually and that portion of the current returned to the power supply. All the elements and processes in the electrospray circuit are in series as shown so that the current that flows in this circuit is everywhere the same.
A circuit that separates the several processes of electrospray ionization is shown in FIG. 2. Most of the processes are shown as functional blocks rather than circuit components to identify the electrical nature of each step. The process in FIG. 2 can be related to the circuit elements in FIG. 1 as follows. The electrochemical contact 15 occurs between the metal to which the power supply is connected and the solution in that region of the metal/solution contact closest to the electrospray tip. If the connection is to a metallic union and a non-metallic glass capillary is used as the spray needle, there may be some solution resistance 16 between the electrochemical contact 15 and the spray tip. At the needle tip, charge separation 17 occurs as a result of the high electric field that exists between the tip and the counter-electrode. The charge separation 17 is in the formation of the charged droplets that emanate from the tip. The charged droplets are then attracted across the air gap 19 between the tip and the counter-electrode. All the charge that is separated at the tip is neutralized 20 at the counter-electrode or inside the mass spectrometer and returned to the power supply 21. Another characteristic of a series circuit is that the sum of voltage drops across all the process shown must equal the voltage applied by the power supply.
High series resistance acts to stabilize the operation of an electrospray ionization device enabling operation over a wider range of experimental conditions than without it. This occurs somewhat naturally in a narrow-bore glass capillary with remove contact. Stability can be achieved with a separate series resistor for glass needles with tip contact, wide-bore glass needles and for metal needles.